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The U.S. Nuclear Regulatory Commission on Tuesday issued a formal Information Notice to nuclear power plant operators warning them about an incident in January in which the Slammer computer worm penetrated networks at Ohio's Davis-Besse nuclear plant and disabled two important monitoring systems for hours.

The notice is the NRC's first public acknowledgement of the incident, which was first reported by SecurityFocus last month.

In a statement, the NRC said it issued the notice to warn utilities of the potential for nuclear plants to be infected by the Slammer worm. But the commission emphasized that the Slammer-induced failures at Davis-Besse did not affect the safe operation of the plant. "NRC regulations require safety-related systems to be isolated or have send-only communication with other systems," the commission said.

The notice follows a private industry advisory circulated last March, and a similarly worded e-mail sent to the NRC in April by an official at FirstEnergy Corp., the Ohio utility that runs the plant. "It just went through the usual bureaucratic process," says NRC spokesman Scott Burnell. "Once the investigations that FirstEnergy had done were completed, they turned it over to us."

The worm entered the Davis-Besse plant by penetrating a contractor's network, then squirming through a T1 line bridging that network and Davis-Besse's corporate LAN. The T1 line, investigators later found, completely bypassed the plant's firewall, which was programmed to block the port Slammer used to spread.

From the corporate network, the worm moved to the plant's operations network, where the traffic jam it produced disabled a system called the Plant Process Computer, and the Safety Parameter Display System -- a computerized display panel that monitors critical plant functions. A redundant analog backup to the SPDS was unaffected by the worm.

The troubled plant had been offline since early 2002, when workers discovered a 6-by-5-inch hole in the plant's reactor head.

The Information Notice includes no specific recommendations, and does not address any danger posed by the more recent Blaster worm. But it notes that the NRC issued a security order in February, 2002, alerting plant operators to the possibility that external network interconnections might improperly bypass protective measures, like firewalls. At FirstEnergy, that security order was implemented by the plant's IT personnel, but was not passed on to the plant computer engineers responsible for the wayward T1 line, according to the notice.

Plant computer engineers were also unaware that a patch was available for the MS-SQL vulnerability that Slammer exploited. Microsoft had released the patch six months earlier.

FirstEnergy has become a focus of the investigation into last month's northeastern U.S. power outage.